# Eli5 what controls if a gas is heavy or not? Example helium Seems to defy gravity while something like xenon I assume would sink

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Eli5 what controls if a gas is heavy or not? Example helium Seems to defy gravity while something like xenon I assume would sink

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The heaviness of a gas is its density, which has a correlation with its molecular mass as a general rule.

Helium doesn’t defy gravity, it’s just floating on air. Air and Helium are both fluids.

Gas is actually really simple to figure this out for. See, at the same temperature and pressure, all gases have about the same number of molecules in a space. An unpressurized 2L bottle of oxygen will have the same number of molecules as a 2L of xenon or helium or air.

So it only depends on how heavy each molecule is, and that you can figure out from the periodic table.

To see if it will float, you just need to know if it’s lighter than air, so let’s see how heavy a molecule of air is. Air is made up of nitrogen and oxygen, both of which exist in molecules of two atoms, so N2 and O2. Nitrogen’s weight on the table is 14 and oxygen’s is 16, so the molecule weights are 28 and 32. Let’s call it 30 for air as a whole, to save some math.

Helium comes in single atoms, and its weight (per the table) is 4, so it’s 4/30 as heavy as air. Hydrogen’s weight is 1, but it also comes in molecules of 2 atoms so its weight is 2.

Xenon, and all noble gases, come in single atoms, and xenon’s weight is 131. *Way* heavier than air.

This 9 bound bowling ball defies gravity. I pushed it to the bottom of my swimming pool, and it just floats straight on up! I thought gravity pushed things down!

When surrounded by other things (air, water, etc) pressing against it, the denser/thicker stuff will sink to the bottom at the expense that the lighter/thinner stuff will be forced to rise upwards. Helium is close to the lightest gas that exists so of course it will want to go up. Xenon is actually quite heavy as an atom and will want to go down. These need to be compared to normal air, which is mostly nitrogen and oxygen, and the difference is very significant.

Hot air balloons take advantage of the same strategy, except that hot air is also thinner than normal/cold air. So a big bag of hot air (and I mean really big bag) will weigh less than a big bag of cold air and float upwards. Then we just tie something to it and go for a ride. Same idea, different technique.

mass of an object is the gravity it exudes and is compelled to.

in empty space is you had a ball of lead, of Styrofoam and a potatoes all the same size the potatoe and lead would pull on each other causing them to move on each other faster than the Styrofoam. eventually the Styrofoam would get to where they meet but it would have to move further as the lead and potatoes would not be as attracted to the foam as each other or themselves.

this is generally referred to as density. you can compress something and its weight per volume would increase but it’s total mass would not change. a bag of feathers that weighs 10 pounds on earth has the same mass as a small 10lb lead weight the reason basically is the have the same number of atomic particles(electrons, neutrons and protons) which varies by the element, the simpler the atom the less particles, the less mass.